This invention relates to substrate coatings containing energy absorbing, temperature stabilizing phase change materials and methods of manufacturing same. More particularly, this invention relates to fabric coatings containing microspheres of phase change material dispersed in a polymer binder and methods of manufacturing same.
Coatings are typically applied to fabrics to increase water resistance, water transport, insulative ability or heat storage properties of the fabrics. Recently, microencapsulated phase change materials have been described as a suitable component for fabric coatings when exceptional heat transfer and storage capabilities are desired. In particular, International Patent Application No. PCT/93/05119 for xe2x80x9cFabric with Reversible Enhanced Thermal Propertiesxe2x80x9d to Colvin, et al., which is incorporated herein by reference, discloses that fabrics coated with a binder containing microcapsules filled with energy absorbing phase change material enables the fabric to exhibit extended or enhanced heat retention or storage properties.
Research has demonstrated that applying a binder containing microspheres of phase change materials with commercial coating equipment can be problematic. For example, use of solvent based gravure printing techniques in which a solvent system was employed to achieve uniform dispersion of the microspheres in a binder proved unsuccessful because the solvent systems damaged the microspheres.
Thermoplastic gravure printing techniques also proved unsatisfactory for use with microspheres of phase change material. When using higher temperature thermoplastic gravure printing techniques, sustained temperature of 325xc2x0 F. caused severe damage to the microspheres. Although lower temperature thermoplastic gravure printing techniques avoided significant damage to the microspheres, the resulting coating was found lacking in washability and durability. Moreover, lower temperature thermoplastic gravure printing techniques precluded addition of the desired amounts of the microspheres, allowing addition of microspheres of up to only about 20% by dry weight of the microsphere/binder material. This low percentage of phase change material in the coating makes the coating susceptible to undesirable heat transfer across the coating, especially in locations where phase change material is sparsely applied.
Attempts to encapsulate microspheres of phase change materials in a thermoplastic spray have also proved unsatisfactory. In particular, scattering microspheres into a stream of sprayed, fibrous thermoplastic material resulted in a binder matrix that did not fully encase the microspheres. The resulting binder/microsphere material was susceptible to loss of microspheres, which worked loose and were continually shedded from the fabric. In addition, the coating lacked uniformity of thickness and microsphere distribution.
Attempts were also made to utilize thermoplastic extrusion techniques to create a film of continuous web in which microspheres of phase change material were uniformly distributed. However, thermal breakdown of the microspheres resulted from the higher temperatures utilized. The extrusion screw employed with these techniques also physically damaged the microspheres.
Phase change materials in microencapsulated form are commonly supplied as a dry powder. This powder is difficult to wet and uniformly disperse in aqueous systems. Moreover, some microencapsulated phase change materials have an internal layer of modified gelatin which is hydrophilic and capable of absorbing its own weight in water. Not only does the hydrophilic quality of such microcapsules make more standard component proportions inapplicable, microcapsules which have absorbed water tend to swell and associate, increasing the viscosity of the coating system above acceptable limits. Although the precise behavior of microcapsules in the coating system which have absorbed water is uncertain, it is believed that such microcapsules agglomerate, reducing their dispersion throughout the binder of the coating system, which de-stabilizes the binder. This de-stabilization can increase over time. When latex binders are used with microencapsulated phase change material, de-stabilization of the latex binder can continue until the latex binder coagulates.
U.S. Pat. Nos. 5,254,380, 5,211,949, 5,282,994 and 5,106,520 for xe2x80x9cDry Powder Mixes Comprising Phase Change Materialsxe2x80x9d describe free flowing, conformable powder-like mixes of silica particles and a phase change material which the silica particles of between 7xc3x9710xe2x88x923 to 7xc3x9710xe2x88x922 microns are mixed with phase change material in a ratio of up to 80% by weight of phase change material. While these patents describe a matrix in which microspheres of phase change materials need not be separately encapsulated, they do not describe the use of dry powder mixes containing phase change materials in binder matrices for coating fabrics.
It is against this background that the significant improvements and advancement of the present invention have taken place in the field of fabric coatings containing energy absorbing, temperature stabilizing phase change materials and methods of manufacturing same.
It is the principal object of the present invention to provide an improved fabric coating composition containing phase change material of a density sufficient to effect or control heat and energy transfer across the coating and/or store heat in the coating.
It is another object of the present invention to provide a coating composition of the foregoing character which will maintain substantially all of the breathability, flexibility or other principal qualities of the fabric to which it is applied.
It is a further object of the present invention to provide coated fabrics having the aforementioned properties which are resistant to heat, pressure and chemicals encountered during the coating process.
It is a still further object of the present invention to provide coated fabrics having the aforementioned qualities which are durable, resistant to heat, moisture, solvents, laundering, and/or drycleaning, without degradation to or loss of the phase change material.
It is still another object of the present invention to provide an improved method of applying coating compositions containing phase change materials and having the aforementioned qualities as coatings on fabrics by utilizing commercially available equipment.
It is yet another object of the present invention to provide an improved method of applying coatings containing phase change materials to fabrics without damage or degradation to the phase change materials.
It is still another object of the present invention to provide an improved method for evenly dispersing phase change material throughout a binder and maintaining an even distribution of the phase change material while coating a fabric with the binder and phase change material dispersion.
The present invention comprises coatings for fabrics and methods for manufacturing the same. A preferred coating includes wetted microspheres containing a phase change material dispersed throughout a polymer latex binder, and including a surfactant, a dispersant, an antifoam agent and a thickener. Preferred phase change materials include paraffinic hydrocarbons. To prepare a preferred coating composition of the present invention, microspheres containing phase change material are dispersed in an aqueous solution of a surfactant, a dispersant, and an antifoam agent mixture, followed by dispersion in a polymer mixture to form a coating composition. An alternative method of preparing the coating composition of the present invention includes dispersing microspheres containing phase change material in wet cake form in an aqueous solution of a surfactant, a dispersant, antifoam agent and polymer to form a coating composition. The coating composition of the present invention are then applied as a coating on a fabric.
In accordance with the present application, it has been discovered that wetting microspheres of phase change materials with water and maintaining a uniform dispersion of the microcapsules in a wet coating minimizes or eliminates the tendency of such microspheres to destabilize the binder polymer in which the microspheres are dispersed.
A coating composition which includes microspheres containing a phase change material is prepared by mixing dry microspheres with an excess of water to induce the microspheres to swell with water until swelling is complete. Preferably, a surfactant and a dispersant are added to the water prior to mixing with the microspheres. The surfactant decreases surface tension of the layers of the microspheres and thereby promotes wetting of the microspheres. An antifoam agent is added to and mixed slowly with the microsphere/water mixture to remove air trapped as dispersed bubbles in the mixture. A thickener is added to adjust the viscosity of the mixture to prevent the microspheres from floating or sinking in the mixture. A viscosity of at least 500 cps is preferred. Adjusting the pH of the mixture to 8.5 or greater promotes swelling of the microspheres. Swelling is typically complete in from 6 to 24 hours, at which time the microspheres will have reached an equilibrium with the aqueous phase in which they are dispersed. Thereafter, the microsphere dispersion is added to a mixture of a polymer dispersion, surfactant and dispersant having a pH approximately the same as the pH of the microsphere dispersion. The viscosity and rheology of the resulting coating compound is adjusted to meet the requirements of the coating method employed.
The polymeric binder may be in the form of a solution, dispersion or emulsion in water or in organic solvent. The polymeric binder may initially be polymeric, or in the form of monomers and/or oligomers, or low molecular weight polymers which upon drying and/or curing are converted to their final molecular weight and structure. These binders are preferably film-forming, elastomeric, and have a glass transition temperature in the range of about xe2x88x9245xc2x0 C. to +45xc2x0 C., depending upon the desired application. For most garment applications, an elastomeric polymer with a glass transition temperature of about xe2x88x9230xc2x0 C. to about +12xc2x0 C. is preferred.
The polymers may be linear or branched. Copolymers may be random, block or radial. The polymers may have pendant reactive groups, reactive ends or other crosslinking mechanisms, or be capable of entanglement and/or hydrogen bonding in order to increase the toughness of the finished coating and/or its resistance to heat, moisture, solvents, laundering, dry-cleaning or other chemicals.
Suitable monomers include, but are not limited to, acrylic eaters (preferably alkylacrylates and methacrylates containing 4 to 17 carbon atoms); styrene; isoprene; acrylonitrile; butadiene; vinyl acetate; vinyl chloride; vinyldiene chloride; ethylene; butylene; propylene; chloroprene; etc. Polymers and copolymers based upon the above mentioned monomers and/or upon silicone; epoxy; polyurethane; fluorocarbons; chlorosulfonated polyethylene; chlorinated polyethylene; and other halogenated polyolefins are also useful.
The surfactant described above has a preferred wetting time of not greater than 50 seconds at a concentration of 0.10 % by the Draves Wetting Test. Nonionic and anionic surfactants are acceptable. Dioctyl sodium sulfosuccinamate (sometimes referred to herein as xe2x80x9cDOSxe2x80x9d) is a preferred surfactant.
The dispersing agent employed as described above is preferably a nonionic or anionic dispersant, such as dispersants based upon phosphate esters. A 90% solution of the potassium salt of a phosphated coester of an alcohol and an aliphatic ethoxylate such as Strodex PK907 available from Dexter Chemical Company of New York City, N.Y. is a preferred dispersant.
From 0.1% to 0.8% by weight of dry DOS to dry microspheres and from 0.1% to 0.8% by weight of dry PK90(trademark) to dry microspheres is effective. The total amount of DOS and PK90(trademark) is preferably apportioned equally between the dry microsphere dispersion and the polymer dispersion to which the microspheres will be added after swelling is complete.
Suitable thickeners include polyacrylic acid, cellulose esters and their derivative, polyvinyl alcohols, and others known in the art. A preferred thickener is Acrysol ASE60(trademark) available from Rohm and Haas Company of Philadelphia, Pa. ASE60(trademark) is preferably obtained as a 28% solution of an alkali-swellable polyacrylic acid which increases in viscosity upon neutralization. As described above, thickener is added first to achieve the desired viscosity of the microsphere dispersion, which will vary depending on the particular phase change material selected, and then to adjust the wet coating to meat the requirements of the coating method employed.
Preferred antifoam agents include aqueous dispersions of silicone oil, such as polydimethylsiloxane, containing dispersed fine particle silica, and mixtures of mineral oil, surfactant and fine particle silica, such as AF 9020(trademark) available from General Electric Company of Waterford, N.Y., and Advantage 831(trademark) available from Hercules Chemical Company of Wilmington, Del.
A preferred polymer binder is made with a dispersed polymer latex is an anionic, heat reactive, acrylic latex containing 59% non-volatiles in water, such as the acrylic polymer latex marketed under the tradename Hycar XT9202(trademark) and available from B. F. Goodrich Chemical Company of Cleveland, Ohio. The polymer latex has a glass transition temperature of xe2x88x9225xc2x0 C. When properly dried and cured, fabric coatings made from polymer latex such as Hycar XT9202(trademark) are washable and dry-cleanable.
The coating compositions of the present invention preferably include from 30 to 500 parts by dry weight of microspheres for each 100 parts by dry weight of acrylic polymer latex. The coating compositions preferably include from 0.005% to 6% dry weight each of surfactant and dispersant to dry weight of microspheres. Water is added to total 25% to 80% of the final wet coating composition. An antifoam agent of from 0% to 1% dry weight to total weight of the final wet coating composition is preferred. The most preferred ratios of components of the coating composition of the present invention are: 70 to 300 parts by dry weight of microspheres for each 100 parts by dry weight of acrylic polymer latex, 0.1% to 1% dry weight each of surfactant and dispersant to dry weight of microspheres, water totaling 40% to 60% of the final wet coating composition and antifoam agent of from 0.1% to 0.5% dry weight to total weight of the final wet coating composition.
An alternative method utilizes microspheres of phase change material which are not completely dried during the manufacturing process. Wet microspheres containing from about 25% to about 65% by weight water are preferred and can be readily handled. When using such microspheres, a surfactant and a dispersant are added to a polymer binder dispersion before the wetted microspheres are dispersed therein. DOS and Strodex PK90(trademark) are preferably mixed with the polymer binder dispersion before the wet microspheres are mixed with and dispersed therein.
Generally speaking, phase change materials have the capability of absorbing or releasing thermal energy to reduce or eliminate heat transfer at the temperature stabilizing range of the particular temperature stabilizing material. The phase change material inhibits or stop the flow of thermal energy through the coating during the time the phase change material is absorbing or releasing heat, typically during the material""s change of phase. This action is transient, i.e., it will be effective as a barrier to thermal energy until the total latent heat of the temperature stabilizing material is absorbed or released during the heating or cooling process. Thermal energy may be stored or removed from the phase change material, and can effectively be recharged by a source of heat or cold. By selecting an appropriate phase change material, a fabric can be coated for use in a particular application where the stabilization of temperatures is desired. Two or more different phase change materials can be used to address particular temperature ranges and such materials can be mixed.
Paraffinic hydrocarbon phase change materials suitable for incorporation into fabric coatings are shown below in Table I. The number of carbon atoms contained in such materials and is directly related to the melting point of such materials.
Phase change materials such as the listed paraffinic hydrocarbons are preferably formed into microspheres and encapsulated in a single or multi-layer shell of gelatin or other materials. Encapsulated microsphere diameters of from 1 to 100 microns are preferred, most preferably in the range 10 to 60 microns. Encapsulated microspheres containing phase change materials are sometimes referred to herein as xe2x80x9cmicroPCMs.xe2x80x9d Microspheres may also be bound in a silica matrix of sub-micron diameters. Microspheres containing n-octadecane or n-eicosane are suitable for fabric coatings for clothing. Such microspheres are available from MacGill Enterprises, Inc. of West Milton, Ohio and Microtek Laboratories, Inc. of Dayton, Ohio.